Process for coating steel

ABSTRACT

PROCESS FOR DEPOSITING CHROMIUM HYDROXIDE ON STEEL IS PROVIDED. A SMALL AMOUNT OF ONE OR MORE OF A THIOSULFATE, DITHIONITE, BISULFITE, SULFITE, THIOSULFITE, PYROSULFITE, SULFIDE AS AN AUXILIARY AGENT IS ADDED TO A MAIN ELECTROLYTE SOLUTION OF CHROMIC ACID AND A CATHODE ELECTROLYTIC REDUCTION THEREOF IS CARRIED OUT IN A CONVENTIONAL CELL.

United States Patent 3,794,572 PROCESS FOR COATING STEEL HidehisaYamagishi and Hiroshi Takano, Kawasaki,

Japan (both Nippon Kokan Kabushiki Kaisha Technical Institute, 2730Miuamiwataridachio, Kawasaki-sh], Kanagawa-ken, Japan) No Drawing. FiledJune 1, 1970, Ser. No. 42,505 Claims priority, application Japan, Apr.28, 1967, 42/27,117 Int. Cl. C23h 11/02 US. Cl. 204-56 R 10 ClaimsABSTRACT OF THE DISCLOSURE Process for depositing chromium hydroxide onsteel is provided. A small amount of one or more of a thiosulfate,dithionite, bisulfite, sulfite, thiosulfite, pyrosulfite, sulfide, as anauxiliary agent is added to a main electrolyte solution of chromic acidand a cathode electrolytic reduction thereof is carried out in aconventional cell.

FIELD OF INVENTION This invention relates to a process for producing acoating having excellent adhesion properties and/or corrosion resistanceusing an ordinary stationary cell and electrolytic reduction with anaqueous treating solution comprising chromic acid as the primary agentand an auxiliary agent selected from thiosulfate, sulfite, dithionite,bisulfite, thiosulfite and sulfide.

BACKGROUND OF INVENTION 'Heretofore, in producing a corrosion resisting-film on the surface of steel with a hydrated chromium oxide as theessential film component, a baking treatment or electrolytic reductionhas been employed. Chromic acid or chromate in relatively lowconcentration in solution has been used as the primary agent, withvarious additives as the auxiliary agents. In electrolytic reduction,typical auxiliary agents which have been used include: sulfuric acid,tartaric acid, maleic acid, oxalic acid, aromatic sul- :fonic acidhaving a hydroxyl group, sucrose, and glycol. Such electrolytic platingsolutions have not been completely satisfactory in that chromium metalwhich is produced in the film is only 70-80% of the total quantity ofchromium deposited, and the remaining chromium of 20- 30% exists ashydrated chromium oxide. Therefore, the coating film produced byconventional electrolytic treatment generally comprises a double layerconsisting of metallic chromium and hydrated chromium oxide layersdeposited on the surface of steel, notwithstanding the difference insome of the electrolytic conditions or the difference in auxiliaryagents. It is true that such a double layer contributes to the corrosionresistance, but problems remain with regard to adhesion or color tone ofthe surface. The object of the present invention is to provide a processfor forming easily a coating which is primarily chromium oxide.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a process for depositing chromium hydroxide on a steelsurface which comprises contacting a steel surface with an aqueouschromium plating solution for from 1 to 5 seconds and a current densityof to 50 A/dm. the solution containing from about 50 to about 100 gramsper liter of chromic acid and from about 0.5 to about 1 gram per literof an auxiliary agent selected from the group consisting of an alkalimetal thiosulfate, sulfite, dithionite, bisulfite, thiosulfite,pyrosulfite, and sulfide.

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SPECIFIC EMBODIMENTS OF THE INVENTION It is a feature of this inventionthat the aqueous chromium solutions employed have an usually lowconcentration of chromic acid. The concentration ranges from about 50 toabout 100 grams per liter (g./l.). Concentrations below about 50 g./l.are insuflicient to provide a suitable film on the steel surfaces whichare treated. More than about 100 g./l. results in considerable depositof metallic chromium, rather than the desired chromium hydroxide. Withexcessive deposits of metallic chromium, the resulting films suffer inadhesion properties.

The aqueous chromium solutions also contain one or more auxiliaryagents. These agents include water-soluble thiosulfates, sulfites,dithionites, bisulfites, thiosulfites, pyrosulfites and sulfides,particularly the alkali metal compounds of these groups. Theconcentration of such an agent or agents in the solutions is alsocontrolled. Such concentrations range from about 0.5 to about 1 g./l. ofsolution. Concentrations of less than about 0.5 g./l. are insufficientto provide the desired characteristics to the steel surfaces which serveas the cathode and which are treated. And concentrations in excess ofabout 1 g./l. cause unsatisfactory surface efiects.

Current density employed ranges from about 10 to about 50 A/dmfi, withabout 30 preferred. Plating or treating time with such a current densityis extremely short, from about 1 to about 5 seconds, with about 3preferred.

In treating a steel surface, it is subjected to conventional cleaningtreatments to remove any fats or oils. It can be contacted with aconventional pickling solution to remove any rust which may be on thesurface, and then washed with water. It is then contacted in aconventional stationary chromium plating cell, under the conditions ofthis invention.

Steel such as strip steel, ordinary carbon steel sheet and the like aresuitable for use in the process of this invention.

The invention is illustrated by the following examples.

EXAMPLE 1 Chromic acids having dilferent concentrations, wereelectrolyzed in a conventional stationary plating cell using specifiedauxiliary agents under the conditions shown in Table 1 below and using aworkpiece as the cathode.

TABLE 1 Concentration of chromic acid, g./l.

Current density,

Auxiliary agent aJdm.

EXAMPLE 2 Samples of Experiment Nos. 1 and 3 in Table 1 and a sampleobtained by Experiment No. 1, using 0.5 g./l. of H instead of Na S O forcomparison, were tested for their physical and chemical properties. Theresults of the tests are shown in Table 2.

TABLE 2 Adhesio o 1: Antiw bending property Antiimpaet Erichsen D D Wtest Synthetic mark Experiment N 0.

Square test N orE.The numerals denote marks of evaluation.

Bending test was carried out in accordance with test method 115 Z 2203and impact test in accordance with test method 115 Z 2242.

In the Erichsen test, the samples, the coated surfaces of which are onthe outside, are pushed out of the inside of an Erichsen tester by acircular rod having a certain diameter at a definite speed, and thedistance by which the samples are pushed is measured when cracks orexfoliations appear.

In the square test, the coated surface of the samples is cut with aknife in such a way as to make 100 squares, each of which has an area of1 mm. so deeply as to reach the matrix, and the state of the exfoliationof the film is observed.

In any of the tests shown in Table 2, values obtained as a result of thetests are converted into evaluated marks for convenience, and thesynthetic marks are obtained by adding all the marks in each experiment.

Table 3 shows the result of chemical property tests.

NOTE-Numerals denote marks of evaluation.

Salt spray test is carried out in accordance with test method JIS Z2371. The result of the test is evaluated as 1 when rust appears in morethan 51% of the area of the surface of the sample, and as 10 when rustdoes not appear (i.e. of the area).

Shade test is carried out with regard to the colors of rusts. A sampleon which the color of the rust does not change at all is evaluated as10, and one whereon the characteristic reddish brown rust appears isevaluated as 1.

As demonstrated with the above-described examples, involving thespecified auxiliary agents of this invention, the anti-corrosiveproperty of a film on the surface of the samples is substantiallysuperior to that of a film wherein H was used in the treating solution.

What is claimed is:

1. A process for depositing chromium hydroxide on a steel surface whichcomprises contacting a steel surface as a cathode with an aqueouschromium plating solution for from about 1 to about 5 seconds and acurrent density of from about 10 to about 50 A/dmfi, the solutioncontaining from about 50 to about grams per liter of chromic acid andfrom about 0.5 to about 1 gram per liter of an auxiliary agent selectedfrom the group consisting of an alkali metal thiosulfate, sulfite,dithionite, bisulfite, thiosulfite, pyrosulfite and sulfide.

2. A process of claim 1, wherein the chromic acid concentration is about50 grams per liter.

3. A process of claim 1, wherein the chromic acid concentration is about100 grams per liter.

4. A process of claim 1, wherein the agent is Na S O 5. A process ofclaim 1, wherein the agent is Na S O 6. A process of claim 1, whereinthe agent is Na SO 7. A process of claim 1, wherein the agent is NaHS0'8. A process of claim 1, wherein the concentration of said agent is fromabout 0.5 to about 0.65 gram per liter.

9. A process of claim 1, wherein the current density is about 30 A/dmfi.

10. A process of claim 1., wherein the time is about 3 seconds.

References Cited UNITED STATES PATENTS 2,976,193 3/1961 Pimbley 148-6212,746,915 5/ 1956 Giesker et a1. 204-56 R 3,486,990 12/1969 Fukuzuka etal. 204-56 R 2,898,250 8/1959 Pimbley 148-621 3,679,554 7/1972 Yamagishiet a1. 204-56 R 3,479,260 11/ 1969 Rauch et a1 204-56 R 2,998,361 8/1961Kitamura 204-56 R 2,812,296 11/ 1957 Neish 204-56 R 1,233,633 7/1917Allen 204-56 R JOHN H. MACK, Primary Examiner R. L. ANDREWS, AssistantExaminer 9 I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 72 Dated February 26, 1974 'lnventor(s) HIDEHISA YAMAGISHI etal It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line 7 Replace "Filed. .42, 505" by the following:

--Continuationin-part of abandoned application Ser. No. 724,991, April29, 1968. This application Jnne'l", 1970, Ser. No. 42,505--.

Signed and sealed this 15th day of August 197A.

(SEAL) Attest:

McCOY M; GIBSON, JR. (2. MARSHALL DANN Attesting Office-r Commissionerof Patents USCOMM-DC 603764 69 w u.s. covznuugwr nmmu; omc: 2 anoan-s:u.

F ORM PO-I 050 (10-69)

